Some clients just need to know if their SEO investment is worthwhile. But how do you measure ROI, or rather, can you prove that there is a return on the investment?
It's a hard question to answer. On the superficial level, you can check Google Analytics and check if there has been an increase of visitors to your site, and if so, where they came from. But typically, there's really no way to prove ROI since there are so many different factors in play, including the increase in traffic, the attribution of that traffic to your actual SEO efforts, and the long-term investment of your SEO efforts.
Additional metrics include gathering prospective callers from the website (a practical solution to measure this would be to put a different phone number on the website versus on print publications) or by simply asking "How did you hear about us?" to people who dial in.
Forum discussion and (which includes a very nifty guide for metrics posted by nethy) continues on High Rankings Forum.